Chem 125 Lecture 684/15/08

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Pericyclic Reactions(in which transition states are “aromatic”)

Cycloadditions: Diels-Alder

Electrocyclic Reactions

13

conrotation disrotationMöbius

2

Hückelconnectivity

requires twistin 1 of 2 ways

Hückel

Transition StateMotion

1

2

3

4

21

3

4

5

6

21

3

4

5

6

1

2

3

4

top touches

top(even #

of nodes)

top touches

bottom(odd # of nodes)

! 21

3

4

5

6

Sec. 27.2pp. 1343-1346

(shown backwards in our text;same favored “aromatic”

conrotatory transition state)

CON4e

CON for 4nCON8e

DIS6e

DIS for 4n+2

2

Transition State HOMO-1

1

disrotation6e Hückel

bottom touches top(odd # of nodes)

top touches top(even # of nodes)

conrotation4e Möbius

Molecules of C4H4 will react with one another unless isolated.

Is cyclobutadiene antiaromatic (4n)?Make it and see.

OO O

OMöbius conrotation

impossible because of product strain

OO

+ O=C=O

h

h

Photochemistry changes electrocyclic preference to disrotation, because an electron is promoted to the next MO

with opposite symmetry.

Photochemistry changes preference of (reverse) cycloaddition too, allowing cyclobutane to become two double bonds.

Make one moleculeper cage

. . ... .. .

..... .

...

.

...

as guestBenzene

Antiaromaticupfield shift?

Most shift comes fromother rings, still ~1.5ppm above benzene

Normal

Making & Studying“antiaromatic”Cyclobutadiene

above center of 8 benzene rings

Opening Dewar Benzene

(1866)

Calculated Isomers of Benzene

(2004)

84 calculated to be < 100 kcal above benzene.

6 > 100 kcal above benzene have been prepared.

Dewar Benzene (1963) is 74 kcal above benzene but lasts 2 days at room temperature!

t1/2 = 2 days (room temp)

-11 kcal

-75 kcal

25

33

conrotatory

disrotatory

more strain

aromatic

66 kcal/mole more exothermic,but only 8 kcal/mole “faster”

But shouldn’t “aromatic” 6--e transition state be good for disrotation?

It is more fundamental that LUMO doesn’t overlap HOMOs (& vice versa).

Making & Studying“antiaromatic”Cyclobutadiene

To discuss making this “clamshell” we’ll

need to learn some more reactions:

Electrophilic Aromatic Substution

and

Carbonyl Reactions

Electrophilic Aromatic

Substitution

Electrophilic Aromatic Substitution

H

HH

H

H

H

D2SO4

H

HH

H

D

H

Sec. 16.4

H

HH

H

H

H

D

via A/D intermediate

+

D2SO4C6D6

etc.

Observable!

Or other electrophiles in place of D+

e.g. NO2+, Br+, HOSO2, R+, R-C=O R-C=O+HOSO2

+

H3C CH3

H3C

H3C

CH3

CH3

CH3

+

A

C C

B

D

B

1H vs. 1Hcorrelation in time

H3C CH3

H3C

H3C

CH3

CH3

CH3

+

CH3

CH3H3C

H3C

CH3

CH3

CH3

+

A

A

C C

B

D

B

C

C

B

D

B

0.3 sec 40°C

CA

DB

Remember

+

+

+

SHMo2 (Simple Hückel Molecular Orbital Program)

Benzene Pentadienyl

Cation addition converts ring to chain.

H

HH

H

H

H

D+

H

HH

H

H

H

D

+

Locus ofodd electron in radical,

+ charge(LUMO)

in cation.

- charge (HOMO)

in anion,SOMO

(nonbonding)

++

+

Cf.

X

NO2

++

+ +

HO2N

X

+ ++

+

H

O2N

X

+

+ +

H

Substituent Effects on RateX

O2N

X

O2N

X X

NO2

+ +NO2

+

(from HONO2/H2SO4)

X Relative Rate (overall)

H [1] Cl 0.03 NO2 6 10-

8

CH3 25 OH 1000

(CH3)3N+ 1 10-

8

donation / withdrawal

(or ) e-donation eases formation of cation intermediates

(or ) e-withdrawal retards formation of cation intermediates

End of Lecture 68April 15, 2009